Inhaled Anesthetic Agents PDF

Inhaled Anesthetic Agents AWAD MOHAMMED ALQAHTANI BSc of Anesthesia Technology King Khalid University, Muhayil Asir  General Anesthesia  G.A. is a medically induced coma and loss of protective reflexes resulting from the administration of one or more...

Inhaled Anesthetic Agents AWAD MOHAMMED ALQAHTANI BSc of Anesthesia Technology King Khalid University, Muhayil Asir  General Anesthesia  G.A. is a medically induced coma and loss of protective reflexes resulting from the administration of one or more anesthetic agents,  A variety of medication may be administrated.  With the overall aim of ensuring sleep, amnesia, analgesia, relaxation of skeletal muscles, and loss of control of reflex of the autonomic nervous system.  The course of a general anesthetic can be divided into three phases: 1. Induction 2. Maintenance 3. Emergence.  Stages of anesthesia  The Guedal’s classification by Athur Ernest Guedal described four stages of anesthesia in 1937 : Stage 1 (Induction) : The period between the initial administration of induction agents and loss of consciousness. Patient can carry on conversation at this time. Stage 2 (Excitement) : The period following loss of consciousness and marked by excited and delirious activity. During this stage respiration and heart rate may become irregular Stage 3 (surgical anesthesia) : During this stage, the skeletal muscles relax, vomiting stops, and respiratory depression occur, the patient unconscious and ready for surgery It has been divided into 4 planes : 1- Eyes initially rolling, then becoming fixed. 2- Loss of corneal and laryngeal reflex. 3- Pupils dilate and loss of light reflex. 4- intercostal paralysis, shallow abdominal respiration. Stage 4 (Overdose) : This stage is lethal without cardiovascular and respiratory support. introduction  Nitrous oxide, chloroform, and ether wasthe first universally accepted general anesthetics  Methoxyflurane and enflurane, two potent halogenated agents, were used for many years in North American anesthesia practice. Five inhalation agents continue to be used in clinical anesthesiology: Nitrous oxide, Halothane, Isoflurane,Desflurane, and Sevoflurane. Inhalation anesthetics, such as halothane and sevoflurane, are particularly useful in the induction of pediatric patients in whom it may be difficult to start an intravenous line Regardless of the patient’s age, anesthesia is often maintained with inhalation agents. Emergence depends primarily upon redistribution from the brain and pulmonary elimination ofthese agents. Pharmacokinetics of Inhalation Anesthetics  What is pharmacokinetics?  sometimes described as what the body does to a drug, refers to the movement of drug into, through, and out of the body—the time course of its absorption, bioavailability, distribution, metabolism, and excretion. 15 MINIMUM ALVEOLAR CONCENTRATION  The minimum alveolar concentration (MAC) of an inhaled anesthetic is a measure of anesthetic potency. It is defined as the concentration of a vapour in the alveoli of the lungs that is needed to prevent movement in 50% of patients in response to surgical stimulus.  MAC is used to compare the strengths of anesthetic vapours. It is expressed as a percentage by volume.  MAC is determined by testing the response of a test population to a standard surgical midline incision.  Roughly 1.3 MAC of any of the volatile anesthetics (eg, for halothane: 1.3 ×0.74% =0.96%) has been found to prevent movement in about 95% of patients (an approximation of the ED 95); 0.3–0.4 MAC is associated with awakening from anesthesia (MAC awake) when the inhaled drug is the only agent maintaining anesthetic. in other words, it is the concentration of gas that we give the patient and he will sleep and not be able to respond to verbal commands. Clinical Pharmacology of Inhalation Anesthetics Nitrous oxide: Nitrous oxide (N2O,laughing gas) is colorless and essentially odorless.Although nonexplosive and nonflammable, nitrous oxide is as capable as oxygen of supporting combustion.  Physical proberties  nitrous oxide is a gas at room temperature and ambient pressure. Itcan be kept as a liquid under pressure because its critical temperature lies above room temperature. Halothane Physical properties: Halothane is a halogenated alkane. The carbon– fluoride bonds are responsible for its nonflammable and nonexplosive nature. Thymol preservative and amber-colored bottles retard spontaneous oxidative decomposition. It is rarely used in the United States. Isoflurane Physical properties: Isoflurane is a nonflammable volatile anesthetic with a pungent ethereal odor.  Rapid increases in isoflurane concentration lead to transient increases in heart rate, arterial blood pressure, and plasma levels of norepinephrine. Isoflurane dilates coronary arteries, but not as potently as nitroglycerin or adenosine.  Desflurane Physical properties: The structure of desflurane is very similar to that of isoflurane. In fact, the only difference is the substitution of a fluorine atom for isoflurane’s chlorine atom. That “minor” change has profound effects on the physical properties of the drug, however. For instance, because the vapor pressure of desflurane at 20°C is 681 mm Hg, at high altitudes (eg, Denver, Colorado) it boils at room temperature. the low solubility of desflurane in blood and body tissues causes a very rapid induction and emergence of anesthesia. Sevoflurane Physical properties: Like desflurane, sevoflurane is halogenated fluorine. Sevoflurane’s solubility in blood is slightly greater than desflurane Nonpungency and rapid increases in alveolar anesthetic concentration make sevoflurane an excellent choice for smooth and rapid inhalation inductions in pediatric and adult patients. In fact, inhalation induction with 4%to 8%sevoflurane in a 50% mixture of nitrous oxide and oxygen can be achieved within 1 min. Questions are welcome Thank You 15  Reference  Clinical Anesthesiology 6th edition 2018 the Author ; G.Morgan. Maged Mikhail and Michael Murray, chapter 8, page : 153 – 173.